Traffic signals have long been used to regulate the flow of traffic at intersections. Generally, traffic signals have relied on timers or vehicle sensors to determine when to change the phase of traffic signal lights, thereby signaling alternating directions of traffic to stop, and others to proceed.
Emergency vehicles, such as police cars, fire trucks and ambulances, are generally permitted to cross an intersection against a traffic signal. Emergency vehicles have typically depended on horns, sirens and flashing lights to alert other drivers approaching the intersection that an emergency vehicle intends to cross the intersection. However, due to hearing impairment, air conditioning, audio systems and other distractions, often the driver of a vehicle approaching an intersection will not be aware of a warning being emitted by an approaching emergency vehicle.
There are presently a number of optical traffic priority systems that permit emergency vehicles to preempt the normal operation of the traffic signals at an intersection in the path of the vehicle to permit expedited passage of the vehicle through the intersection. These optical traffic priority systems permit a code to be embedded into an optical communication to identify each vehicle and provide security. Such a code can be compared to a list of authorized codes at the intersection to restrict access by unauthorized users. However, the various optical traffic priority systems are incompatible because the vehicle identification code for each of the various optical traffic priority systems is embedded in the optical communication using incompatible modulation schemes.
Generally, an optical traffic priority system using a particular modulation scheme is independently purchased and implemented in each jurisdiction, such as a city. Thus, the traffic lights and the emergency vehicles for the jurisdiction are equipped to use the particular modulation scheme. However, a neighboring jurisdiction may use equipment that embeds the vehicle identification code using an incompatible modulation scheme. Frequently, a pursuit by a police car or the route of an ambulance may cross several jurisdictions each using an incompatible modulation scheme to embed the vehicle identification information. It may be burdensome and expensive to allow a vehicle from a neighboring jurisdiction to preempt traffic lights while maintaining appropriate security to prevent unauthorized preemption of traffic lights.